1. Field of the Invention
The present invention relates to an intake system for an internal combustion engine. More specifically, the invention relates to an intake system having two swirl intake ports on each cylinder of the engine.
2. Description of the Related Art
An intake system in which two swirl intake ports are disposed on each cylinder of the engine in order to generate a strong swirl of intake air in the cylinder is known in the Art. For example, this type of the intake system is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-158459. The intake system in the '849 publication disposes two helical type swirl ports on the cylinder. The helical type swirl port has a helical air passage which gives a rotational velocity to the intake air flowing therethrough. In the '849 publication, the swirl port located upstream with respect to the flow direction of the swirl in the cylinder rotates the flow of intake air therethrough by a large amount before it flows into the cylinder while the swirl port located downstream with respect to the swirl in the cylinder rotates the flow of intake air therethrough by only a half turn before it flows into the cylinder.
In the '849 publication, the helical air passage of the upstream swirl port has a relatively large height at the end thereof in order to rotate the flow of intake air by a large amount. In contrast to this, the helical air passage of the downstream swirl port has a relatively small height at the end thereof in order to rotate the flow of intake air therethrough by only a half turn.
By setting the amount of rotation of the flow of intake air through the downstream swirl port small, the tangential (rotational) velocity, i.e., a velocity in the horizontal direction (i.e., the direction perpendicular to the axis of the cylinder), of the intake air at the outlet of the downstream swirl port becomes smaller than the tangential velocity of the upstream swirl port. Therefore, a downward velocity component becomes dominant in the flow of intake air from the downstream swirl port, and the intake air from the downstream swirl port flows downward from the downstream swirl port and is subject to less interference with the cylinder wall. Further, since the flow of the intake air from the downstream swirl port has smaller tangential velocity, the intake air from the downstream swirl port hardly interferes with the swirl of intake air generated by the upstream swirl port. Therefore, the swirl in the cylinder is not disturbed or weakened by interference with the flow of intake air from the downstream swirl port.
In the '849 publication, the amount of rotation of the flow of intake air through the downstream swirl port is kept small by setting the height of the end portion of the helical air passage thereof small. However, in the actual intake system, it is difficult to keep the amount of rotation of intake air small only by setting the height of the end portion of the helical air passage small. Therefore, in some cases system in the '849 publication, the flow of intake air from the downstream swirl port interferes with the cylinder wall and the swirl in the cylinder. Such interference causes an increase in the flow resistance of the downstream swirl port and a decrease in the amount of the intake air therethrough. Further, this interference between intake air from the downstream swirl port and the swirl in the cylinder weakens the swirl. This adversely affect the combustion in the cylinder.